Actually. It's kind of just Spike Lee complaining that Quentin Tarantino made a movie about slavery, considering Quentin's lack of "seriousness" in his films Spike Lee feels a movie like Django Unchained is disrespectful to the history of American slavery

There have been many reported media outlets that have criticized Django Unchained, Spike Lee may be one of the more prominent figures to have publicly displayed his displeasure with the movie (and subsequently with Tarantino) but many reviewers and even normal people have shown disdain for the film as well.